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Signals

Explore the implementation of signal handlers in UNIX, handling program faults and external events efficiently. Learn about signal-driven socket processing, signal masking, and setting signal handlers for optimal program responsiveness.

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Signals

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  1. Signals

  2. Problems • There is a very complicated computation that takes a very very long time, in order to let the user know the program is working hard on the computation instead of hanging, we want to print a “.” on the screen every second while doing the computation. Csc352-Summer03, Stanley Yao

  3. Problems (cont.) • In your program, you want to do some computation and also want to receive socket packages coming from the network. You don’t want your program stupidly sitting there solely listening to the socket port. Instead, you want it to do the computation while there is no socket packages coming in, and whenever a package comes in you put the computation aside for a while and process the package first. Csc352-Summer03, Stanley Yao

  4. Signal • A UNIX mechanism to notify a process about program faults and external events. • Program faults: divided by zero, segmentation violation, bus error, etc. • External events: process termination, I/O available, broken pipe, etc. • Signals: Many different signals identified by a unique integer and represent a particular event • Signal handler: in a process, for each signal, there is a function that is called automatically by the system when the signal is received by the process. You can register your own singal handler, otherwise a default one will be used. Csc352-Summer03, Stanley Yao

  5. Signal Csc352-Summer03, Stanley Yao

  6. signal signal signal Process Process Process SIGINT handler SIGINT handler SIGINT handler SIGQUIT handler SIGQUIT handler SIGQUIT handler SIGKILL handler SIGKILL handler SIGKILL handler … … … Big Picture OS Csc352-Summer03, Stanley Yao

  7. Setting Signal Handler • typedef void (*sighandler_t)(int); • sighandler_t signal(int signum, sighandler_t handler); • Install a new signal handler for the specified signal and returns the previous signal handler. • handler • SIG_IGN: ignore • SIG_DFL: default action Csc352-Summer03, Stanley Yao

  8. Signal Mask • int sigprocmask(int how, const sigset_t *set, sigset_t *oset); • Change the signal mask and return the old mask by oset (if oset is not NULL) • how • SIG_BLOCK: add set into the mask • SIG_UNBLOCK: remove set from the mask • SIG_SETMASK: replace the current mask with set Csc352-Summer03, Stanley Yao

  9. Signal-driven Socket ServerLoop Queue of datagrams ServerLoop SIGIOHandler Process SocketReceiveBuffer SocketReceiveBuffer Kernel Csc352-Summer03, Stanley Yao

  10. Further Readings • Signals • Richard Stevens, Advanced Programming in the UNIX Environment, Addison-Wesley, 1992, ISBN 0-201-56317-7. • Sample programs: http://www.kohala.com/start/apue.html • Sockets • Richard Stevens, UNIX Network Programming, Volume 1, Second Edition: Networking APIs: Sockets and XTI, Prentice Hall, 1998, ISBN 0-13-490012-X. • Sample programs: http://www.kohala.com/start/unpv12e.html Csc352-Summer03, Stanley Yao

  11. Acknowledgement • John H. Hartman, Classnotes for Csc352-Spring03, CS Dept., University of Arizona, 2003 • Gail Anderson, Paul Anderson, The Unix C Shell Field Guide, Prentice Hall, 1986 • Richard Stevens, UNIX Network Programming, Volume 1, Second Edition: Networking APIs: Sockets and XTI, Prentice Hall, 1998, ISBN 0-13-490012-X Csc352-Summer03, Stanley Yao

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